Integration with dataclasses and attrs

SQLAlchemy 1.4 has limited support for ORM mappings that are established against classes that have already been pre-instrumented using either Python’s built-in dataclasses library or the attrs third party integration library.

Tip

SQLAlchemy 2.0 will include a new dataclass integration feature which allows for a particular class to be mapped and converted into a Python dataclass simultaneously, with full support for SQLAlchemy’s declarative syntax. Within the scope of the 1.4 release, the @dataclass decorator is used separately as documented in this section.

Applying ORM Mappings to an existing dataclass

The dataclasses module, added in Python 3.7, provides a @dataclass class decorator to automatically generate boilerplate definitions of common object methods including __init__(), __repr()__, and other methods. SQLAlchemy supports the application of ORM mappings to a class after it has been processed with the @dataclass decorator, by using either the registry.mapped() class decorator, or the registry.map_imperatively() method to apply ORM mappings to the class using Imperative.

New in version 1.4: Added support for direct mapping of Python dataclasses

To map an existing dataclass, SQLAlchemy’s “inline” declarative directives cannot be used directly; ORM directives are assigned using one of three techniques:

The general process by which SQLAlchemy applies mappings to a dataclass is the same as that of an ordinary class, but also includes that SQLAlchemy will detect class-level attributes that were part of the dataclasses declaration process and replace them at runtime with the usual SQLAlchemy ORM mapped attributes. The __init__ method that would have been generated by dataclasses is left intact, as is the same for all the other methods that dataclasses generates such as __eq__(), __repr__(), etc.

Mapping dataclasses using Declarative With Imperative Table

An example of a mapping using @dataclass using Declarative with Imperative Table (a.k.a. Hybrid Declarative) is below. A complete Table object is constructed explicitly and assigned to the __table__ attribute. Instance fields are defined using normal dataclass syntaxes. Additional MapperProperty definitions such as relationship(), are placed in the __mapper_args__ class-level dictionary underneath the properties key, corresponding to the Mapper.properties parameter:

from __future__ import annotations

from dataclasses import dataclass, field
from typing import List, Optional

from sqlalchemy import Column, ForeignKey, Integer, String, Table
from sqlalchemy.orm import registry, relationship

mapper_registry = registry()


@mapper_registry.mapped
@dataclass
class User:
    __table__ = Table(
        "user",
        mapper_registry.metadata,
        Column("id", Integer, primary_key=True),
        Column("name", String(50)),
        Column("fullname", String(50)),
        Column("nickname", String(12)),
    )
    id: int = field(init=False)
    name: Optional[str] = None
    fullname: Optional[str] = None
    nickname: Optional[str] = None
    addresses: List[Address] = field(default_factory=list)

    __mapper_args__ = {  # type: ignore
        "properties": {
            "addresses": relationship("Address"),
        }
    }


@mapper_registry.mapped
@dataclass
class Address:
    __table__ = Table(
        "address",
        mapper_registry.metadata,
        Column("id", Integer, primary_key=True),
        Column("user_id", Integer, ForeignKey("user.id")),
        Column("email_address", String(50)),
    )
    id: int = field(init=False)
    user_id: int = field(init=False)
    email_address: Optional[str] = None

In the above example, the User.id, Address.id, and Address.user_id attributes are defined as field(init=False). This means that parameters for these won’t be added to __init__() methods, but Session will still be able to set them after getting their values during flush from autoincrement or other default value generator. To allow them to be specified in the constructor explicitly, they would instead be given a default value of None.

For a relationship() to be declared separately, it needs to be specified directly within the Mapper.properties dictionary which itself is specified within the __mapper_args__ dictionary, so that it is passed to the constructor for Mapper. An alternative to this approach is in the next example.

Mapping dataclasses using Declarative Mapping

The fully declarative approach requires that Column objects are declared as class attributes, which when using dataclasses would conflict with the dataclass-level attributes. An approach to combine these together is to make use of the metadata attribute on the dataclass.field object, where SQLAlchemy-specific mapping information may be supplied. Declarative supports extraction of these parameters when the class specifies the attribute __sa_dataclass_metadata_key__. This also provides a more succinct method of indicating the relationship() association:

from __future__ import annotations

from dataclasses import dataclass, field
from typing import List

from sqlalchemy import Column, ForeignKey, Integer, String
from sqlalchemy.orm import registry, relationship

mapper_registry = registry()


@mapper_registry.mapped
@dataclass
class User:
    __tablename__ = "user"

    __sa_dataclass_metadata_key__ = "sa"
    id: int = field(
        init=False, metadata={"sa": Column(Integer, primary_key=True)}
    )
    name: str = field(default=None, metadata={"sa": Column(String(50))})
    fullname: str = field(default=None, metadata={"sa": Column(String(50))})
    nickname: str = field(default=None, metadata={"sa": Column(String(12))})
    addresses: List[Address] = field(
        default_factory=list, metadata={"sa": relationship("Address")}
    )


@mapper_registry.mapped
@dataclass
class Address:
    __tablename__ = "address"
    __sa_dataclass_metadata_key__ = "sa"
    id: int = field(
        init=False, metadata={"sa": Column(Integer, primary_key=True)}
    )
    user_id: int = field(
        init=False, metadata={"sa": Column(ForeignKey("user.id"))}
    )
    email_address: str = field(
        default=None, metadata={"sa": Column(String(50))}
    )

Mapping dataclasses using Imperative Mapping

As described previously, a class which is set up as a dataclass using the @dataclass decorator can then be further decorated using the registry.mapped() decorator in order to apply declarative-style mapping to the class. As an alternative to using the registry.mapped() decorator, we may also pass the class through the registry.map_imperatively() method instead, so that we may pass all Table and Mapper configuration imperatively to the function rather than having them defined on the class itself as class variables:

from __future__ import annotations

from dataclasses import dataclass
from dataclasses import field
from typing import List

from sqlalchemy import Column
from sqlalchemy import ForeignKey
from sqlalchemy import Integer
from sqlalchemy import MetaData
from sqlalchemy import String
from sqlalchemy import Table
from sqlalchemy.orm import registry
from sqlalchemy.orm import relationship

mapper_registry = registry()

@dataclass
class User:
    id: int = field(init=False)
    name: str = None
    fullname: str = None
    nickname: str = None
    addresses: List[Address] = field(default_factory=list)

@dataclass
class Address:
    id: int = field(init=False)
    user_id: int = field(init=False)
    email_address: str = None

metadata_obj = MetaData()

user = Table(
    'user',
    metadata_obj,
    Column('id', Integer, primary_key=True),
    Column('name', String(50)),
    Column('fullname', String(50)),
    Column('nickname', String(12)),
)

address = Table(
    'address',
    metadata_obj,
    Column('id', Integer, primary_key=True),
    Column('user_id', Integer, ForeignKey('user.id')),
    Column('email_address', String(50)),
)

mapper_registry.map_imperatively(User, user, properties={
    'addresses': relationship(Address, backref='user', order_by=address.c.id),
})

mapper_registry.map_imperatively(Address, address)

Using Declarative Mixins with Dataclasses

In the section Composing Mapped Hierarchies with Mixins, Declarative Mixin classes are introduced. One requirement of declarative mixins is that certain constructs that can’t be easily duplicated must be given as callables, using the declared_attr decorator, such as in the example at Mixing in Relationships:

class RefTargetMixin:
    @declared_attr
    def target_id(cls):
        return Column("target_id", ForeignKey("target.id"))

    @declared_attr
    def target(cls):
        return relationship("Target")

This form is supported within the Dataclasses field() object by using a lambda to indicate the SQLAlchemy construct inside the field(). Using declared_attr() to surround the lambda is optional. If we wanted to produce our User class above where the ORM fields came from a mixin that is itself a dataclass, the form would be:

@dataclass
class UserMixin:
    __tablename__ = "user"

    __sa_dataclass_metadata_key__ = "sa"

    id: int = field(
        init=False, metadata={"sa": Column(Integer, primary_key=True)}
    )

    addresses: List[Address] = field(
        default_factory=list, metadata={"sa": lambda: relationship("Address")}
    )


@dataclass
class AddressMixin:
    __tablename__ = "address"
    __sa_dataclass_metadata_key__ = "sa"
    id: int = field(
        init=False, metadata={"sa": Column(Integer, primary_key=True)}
    )
    user_id: int = field(
        init=False, metadata={"sa": lambda: Column(ForeignKey("user.id"))}
    )
    email_address: str = field(
        default=None, metadata={"sa": Column(String(50))}
    )


@mapper_registry.mapped
class User(UserMixin):
    pass


@mapper_registry.mapped
class Address(AddressMixin):
    pass

New in version 1.4.2: Added support for “declared attr” style mixin attributes, namely relationship() constructs as well as Column objects with foreign key declarations, to be used within “Dataclasses with Declarative Table” style mappings.

Applying ORM mappings to an existing attrs class

The attrs library is a popular third party library that provides similar features as dataclasses, with many additional features provided not found in ordinary dataclasses.

A class augmented with attrs uses the @define decorator. This decorator initiates a process to scan the class for attributes that define the class’ behavior, which are then used to generate methods, documentation, and annotations.

The SQLAlchemy ORM supports mapping an attrs class using Declarative with Imperative Table or Imperative mapping. The general form of these two styles is fully equivalent to the Mapping dataclasses using Declarative Mapping and Mapping dataclasses using Declarative With Imperative Table mapping forms used with dataclasses, where the inline attribute directives used by dataclasses or attrs are unchanged, and SQLAlchemy’s table-oriented instrumentation is applied at runtime.

The @define decorator of attrs by default replaces the annotated class with a new __slots__ based class, which is not supported. When using the old style annotation @attr.s or using define(slots=False), the class does not get replaced. Furthermore attrs removes its own class-bound attributes after the decorator runs, so that SQLAlchemy’s mapping process takes over these attributes without any issue. Both decorators, @attr.s and @define(slots=False) work with SQLAlchemy.

Mapping attrs with Declarative “Imperative Table”

In the “Declarative with Imperative Table” style, a Table object is declared inline with the declarative class. The @define decorator is applied to the class first, then the registry.mapped() decorator second:

from __future__ import annotations

from typing import List

from attrs import define
from sqlalchemy import Column
from sqlalchemy import ForeignKey
from sqlalchemy import Integer
from sqlalchemy import MetaData
from sqlalchemy import String
from sqlalchemy import Table
from sqlalchemy.orm import registry
from sqlalchemy.orm import relationship

mapper_registry = registry()


@mapper_registry.mapped
@define(slots=False)
class User:
    __table__ = Table(
        "user",
        mapper_registry.metadata,
        Column("id", Integer, primary_key=True),
        Column("name", String(50)),
        Column("fullname", String(50)),
        Column("nickname", String(12)),
    )
    id: int
    name: str
    fullname: str
    nickname: str
    addresses: List[Address]

    __mapper_args__ = {  # type: ignore
        "properties": {
            "addresses": relationship("Address"),
        }
    }

@mapper_registry.mapped
@define(slots=False)
class Address:
    __table__ = Table(
        "address",
        mapper_registry.metadata,
        Column("id", Integer, primary_key=True),
        Column("user_id", Integer, ForeignKey("user.id")),
        Column("email_address", String(50)),
    )
    id: int
    user_id: int
    email_address: Optional[str]

Note

The attrs slots=True option, which enables __slots__ on a mapped class, cannot be used with SQLAlchemy mappings without fully implementing alternative attribute instrumentation, as mapped classes normally rely upon direct access to __dict__ for state storage. Behavior is undefined when this option is present.

Mapping attrs with Imperative Mapping

Just as is the case with dataclasses, we can make use of registry.map_imperatively() to map an existing attrs class as well:

from __future__ import annotations

from typing import List

from attrs import define
from sqlalchemy import Column
from sqlalchemy import ForeignKey
from sqlalchemy import Integer
from sqlalchemy import MetaData
from sqlalchemy import String
from sqlalchemy import Table
from sqlalchemy.orm import registry
from sqlalchemy.orm import relationship

mapper_registry = registry()

@define(slots=False)
class User:
    id: int
    name: str
    fullname: str
    nickname: str
    addresses: List[Address]

@define(slots=False)
class Address:
    id: int
    user_id: int
    email_address: Optional[str]

metadata_obj = MetaData()

user = Table(
    'user',
    metadata_obj,
    Column('id', Integer, primary_key=True),
    Column('name', String(50)),
    Column('fullname', String(50)),
    Column('nickname', String(12)),
)

address = Table(
    'address',
    metadata_obj,
    Column('id', Integer, primary_key=True),
    Column('user_id', Integer, ForeignKey('user.id')),
    Column('email_address', String(50)),
)

mapper_registry.map_imperatively(User, user, properties={
    'addresses': relationship(Address, backref='user', order_by=address.c.id),
})

mapper_registry.map_imperatively(Address, address)

The above form is equivalent to the previous example using Declarative with Imperative Table.